Field of the Invention
This invention relates to the field of polymer coated stents. More specifically, this invention relates to coating treatments for polymer coated stents and delivery balloons to prevent or reduce deployment damage caused by the balloon to a polymer coated stent.
Description of the State of the Art
Stents are being modified to provide drug delivery capabilities. A polymeric carrier, impregnated with a drug or therapeutic substance is coated on the surfaces of a stent. The conventional method of coating is by applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. The dipping or spraying of the composition onto the stent can result in a complete coverage of all stent surfaces, i.e., both luminal (inner) and abluminal (outer) surfaces, with a coating. However, having a coating on the luminal surface of the stent can have a detrimental impact on the stent's deliverability as well as the coating's mechanical integrity.
Briefly, an inflatable balloon of a catheter assembly is inserted into a hollow bore of a stent. The stent is securely crimped on the balloon. The balloon is inflated to implant the stent, deflated, and then withdrawn out from the bore of the stent. A polymeric coating can increase the coefficient of friction between the stent and the balloon of a catheter assembly on which the stent is crimped for delivery. Additionally, some polymers have a “sticky” or “tacky” consistency. If the polymeric material either increases the coefficient of friction or adherers to the catheter balloon, the effective release of the stent from the balloon after deflation can be compromised. If the stent coating adheres to the balloon, the coating, or parts thereof, can be pulled off the stent during the process of deflation and withdrawal of the balloon following the placement of the stent. Adhesive, polymeric stent coatings can also experience extensive balloon sheer damage post-deployment, which could result in a thrombogenic stent surface and possible embolic debris. The stent coating can stretch when the balloon is expanded and may delaminate as a result of such shear stress. Accordingly, there is a need to eliminate or minimize damage caused to a coating of a stent by the delivery balloon.